More particularly, such a device serves to provide an elastic connection between two parts for transmitting static forces from one to the other along the axis of the device, while simultaneously filtering the transmission of associated dynamic forces along the axis from one of the parts to the other.
Although said device can be used in any situation where it is important to filter vibration transmitted between two parts, it is particularly applicable to improving an aircraft suspension device for a gearbox for transmitting engine torque to a driven member, and in particular a main transmission gearbox of a helicopter.
Problems of vibration arise in a helicopter in very specific manner. The main rotor for providing the helicopter with lift and propulsion naturally constitutes a powerful generator of vibration that is transmitted to the fuselage, with the level of the vibration conditioning firstly comfort in the cabin and vibration of the controls, and secondly conditioning the levels of dynamic mechanical stresses throughout the machine. As a result, the dynamic stresses impact on the fatigue lifetime of the parts and the various items of equipment, and consequently the lifetime of the various elements constituting the helicopter.
A large amount of research has been undertaken at least to attenuate, if not completely eliminate, this level of vibration that is inherent to the very operation of the rotor which transmits to the hub not only static or quasi-static forces and moments created by:                lift (perpendicular to the plane of the rotor);        drag (in the plane of the rotor and parallel to the component VH′ of the forward speed of the helicopter that is normal to the rotor mast); and        drift force (perpendicular to the two preceding forces and also in the plane of the rotor) which remains small and can generally be ignored;but also periodic forces and moments coming from aerodynamic asymmetries (profile drag and lift) that appear during rotation of the blades, essentially due to the forward speed in translation flight at high speed, or also to asymmetries that result from unequal distribution of speeds induced on the disk of the rotor at low speed (transition zone). These alternating aerodynamic forces and moments are transmitted to the center of the rotor after being attenuated or amplified naturally by the blades.        
Writing the speed of rotation of the rotor in general manner as Ω, expressed in revolutions per second, and writing the number of blades as b, it is recalled that:                the forces (due to the flapping movements of the blades) and the moments (due to the drag movements of the blades) whose axes are carried by the rotor axis, are transmitted to the mast and to the fuselage only if their frequency, expressed in hertz (Hz) is a harmonic of bΩ, and thus of the form kbΩ (where k is a positive integer equal to or greater than 1), with these forces and moments of the rotating axes being transferred to the stationary axes without any change of frequency (pumping and twisting effects in the structure); and        the forces (due to the drag movements of the blades) and the moments (due to the flapping movements of the blades) of axes lying in the plane of the rotor, are transmitted to the mast and to the fuselage only if their frequency is of the form (kb±1)Ω, with the resulting forces and moments then being at a frequency kbΩ about stationary axes (rolling and pitching effects, transverse or longitudinal shaking, mainly in bΩ).        
Whatever the origin of the excitation, it can thus be seen that a well-balanced rotor transmits to the fuselage, in addition to static forces and moments, only those dynamic forces and moments that are harmonics of bΩ (k=1, 2, 3, . . . ), and that in most cases it is excitation at the first harmonic bΩ that predominates.
In practice, the vibration that is the most troublesome and the most penalizing in a fuselage is vibration in planes that are substantially vertical. Consequently, devices for filtering the transmission of vibration between the rotor and the fuselage of a helicopter are designed in such a manner as to minimize, or even cancel, the following torsor;                a substantially vertical dynamic force, also referred to as “pumping”; and        dynamic moments in pitching (or “longitudinally”) and in rolling (or “laterally”).        
French patent No. 2 499 505 in the name of the Applicant discloses a suspension device for the main transmission gearbox of a helicopter that is effective in filtering such vibration generated by the main rotor providing lift and propulsion.
To this end, the device described in that document is of the type comprising a suspension plate that is situated substantially in a plane perpendicular to the axis of the rotor and having a central portion secured to the bottom of the transmission gearbox, and a set of at least three support bars having top ends connected to the top of the transmission gearbox and bottom ends hinged to the outside ends of arms extending radially around the base of the transmission gearbox, to which they are connected and also, in the region of their outer ends, to the fuselage at strong points thereof. The various connections of each radial arm enable it to move by deformation in the radial plane that contains it, with this movement imparting large displacements to a resonator means constituted by a vibrating mass mounted at the end of a blade serving as a support and whose other end is secured to the corresponding radial arm. Otherwise the suspension plate is independent of the radial arms and is constituted by a thin sheet-metal diaphragm that is deformable in bending under forces exerted perpendicularly to its plane and/or pivot moments exerted around its center by the transmission gearbox, but rigid in traction/compression and in shear under forces and moments exerted in its own plane, such as the torque in reaction to driving the rotor, said diaphragm being integrated in the top structure of the fuselage.
Although particularly effective, said prior art suspension device presents certain drawbacks, and in particular:                it is of limited accessibility and it is bulky, in particular because of the presence of the above-mentioned resonator means; and        its cost is high, in particular because of the specific arrangement for enabling the suspension blades and the mass supports to operate effectively.        
French patent document No. 2 787 161 discloses an anti-vibration device suitable for being mounted between the main transmission gearbox and the fuselage of a helicopter to filter the transmission of vibration between them.
In particular, that anti-vibration device enables the vibrating masses of the kind mentioned above to be eliminated, and it can be installed as a replacement for each of the above-mentioned support bars.
To this end, that prior art vibration filter device comprises:                a resilient annular element connected firstly to the top portion of the transmission gearbox and secondly to the fuselage in such a manner as to act as a connection therebetween, and suitable for being deformed under the action of vibration generated by the main rotor; and        resonator means connected via at least two blades which are flexible in bending and rigid in traction and in compression, to the inside face of said annular element in such a manner as to be set into oscillating pivoting motion during deformation of said annular element produced by the action of said vibration, such that by said oscillating pivoting motion, said resonator means generates an anti-vibratory inertia force opposing said vibration, thereby reducing the transmission thereof from the rotor to the fuselage.        
Nevertheless, that prior art anti-vibration device also presents certain drawbacks:                its mass is significant, since the resonator means comprises vibrating masses connected to the annular element via rigid supports working in bending and blades that are flexible in bending and rigid in traction and in compression. The bending stresses of the supports lead to large dimensions and consequently to high mass for said supports, which are generally made of steel;        the flexible blades are relatively fragile because of the way in which they operate, in particular because of the high stresses due to the dynamic forces and moments, and also because of the static stresses, which can lead to said blades having short lifetimes; and        its amplification ratio λ, characterizing the effectiveness of the device, is contained within small limits.        